Adoption is an experiment in nature in which an individual who derives his genetic endowment from one family receives his environmental and experiential interactions as a member of another family, thus making it possible by appropriate methods to disentangle these two sets of variables to a considerable extent and separately to evaluate their respective roles in the interactions between them. It is the aim of the proposed research to use this device to clarify certain of the genetic and experiential issues which have arisen in the study of the affective illnesses and to contribute to a more valid nosology of these disorders.